1. Field of the Invention
The present invention relates to a light emitting layer for an electroluminescent (EL) device and the electroluminescent employing the same, and more particularly to an improved light emitting layer and an EL device with improved color purity as a result of using metal nanoparticles.
2. Description of the Related Art
Generally, an organic EL device includes a cathode, an organic emission layer, and an anode. Electrons from the cathode and holes from the anode recombine with each other at the organic emission layer to generate excitons. When the excitons are changed from an excited state to a ground state, light emission occurs.
Eastman Kodak Co. developed a multilayer organic EL device including an aluminum quinolinol complex layer and a triphenylamine derivative layer. A use of an organic emission layer including a low molecular weight substance enables broad bandwidth emission ranging from ultraviolet to infrared emission.
Cambridge University reported development of an organic EL device using a polyphenylenevinylene (PPV) film formed by wet coating of conjugated polymer PPV as a light emitting layer. Subsequent to the report by Cambridge University, Covion Co. developed, as light emitting materials, novel PPVs that are soluble in an organic solvent and have good chemical stability and emission efficiency, and Dow Chemical Co. developed polyfluorenes which have good emission property relative to the PPVs. In particular, polyfluorenes have been taken into account as promising blue light-emitting polymers. However, since color purity characteristics are unsatisfactory due to excimers formed by side chains of polyfluorene moieties, there still remains a need for improvement in polyfluorenes based light-emitting materials (U.S. Pat. No. 5,900,327). To improve the color purity characteristics, the conjugated polymer containing 9,9′-spirobifluorene skeleton (U.S. Pat. No. 5,621,131), the conjugated polymer containing a plurality of spiro centers (U.S. Pat. No. 5,763,636), and the conjugated polymers containing heterospiro atoms (U.S. Pat. No. 5,859,211) were developed as light-emitting materials. However, light-emitting materials developed hitherto still exhibit, in particular, unsatisfactory blue emission efficiency and color purity when used in organic EL devices.
Meanwhile, an organic emission layer of an organic EL device includes an organic light-emitting polymer. The photoluminescent (PL) spectrum of such an organic light-emitting polymer is different from the EL spectrum. That is, when excitons are generated while positive and negative charges are migrating toward opposite electrodes, an excimer level is created at a lower energy level than a singlet exciton. The creation of the excimer level induces energy confinement, which prevents an energy transfer. As a result, emission efficiency and color purity are lowered.